Preparation of size and habit-controlled nano crystallites of tungsten oxide

An aqueous sol dispersing nano crystallites of WO3·2H2O was subjected to a centrifugal treatment for 1 or 10 h, to obtain two kinds of WO3·2H2O gels with very different morphology of crystals. The WO3 thick film devices fabricated with these gels were investigated for several properties including NO2 sensing characteristics. It was found that the devices prepared under proper conditions exhibited excellent NO2 sensing properties to cover environmental standard of NO2. The sensitivity to NO2 as well as the rate of response depended markedly on calcination temperature of the devices. The morphology of WO3 crystals, i.e. lamellae or disks, was almost the same as that of WO3·2H2O gel unless calcination temperature exceeded 500 °C. Based on the surface analysis using N2 adsorption technique, the above changes were estimated to result from a change in microstructure of the WO3 crystals.